False-twist crimping machines operate today at thread speeds greater than 600 meters per minute. However, since the temperature of the heating devices utilized in such machines cannot be increased above certain values for reasons concerned with technical process involved, and since the residence time of the yarn within the heating zone cannot be otherwise shortened, it has become necessary to use heaters having increasingly greater lengths. As a result of these developments, false-twist crimping machines have become very large in their overall size.
Several methods have been proposed in order to reduce the size of such machines made necessary by the increased length of their heater elements. For example, British Pat. No. 1,199,071 leads the yarn over the heating element several times. However, such deflection of hot yarn in the false-twisting zone is technically feasible only if special steps are taken to prevent damage to the yarn and yarn twist blockage at the deflection points. It is also proposed in this British Patent to arrange a second heater horizontally above a service passage located between the side frame delivery bobbin and the middle frame. However, the ease of serviceability of the machine suffers by such an arrangement and there is an added danger of vapor collecting in the zone of the horizontal heater.